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From: Trevor Gross <tmgross@umich.edu>
Date: Mon, 5 Feb 2024 15:02:46 -0600
Message-ID: <CALNs47t63qAU2OpokvN6KykT0_NKpDLTBQGhLm-+zsvR1pcUAQ@mail.gmail.com>
Subject: Re: [PATCH] rust: stop using ptr_metadata feature
To: Alice Ryhl <aliceryhl@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>, Wedson Almeida Filho <wedsonaf@gmail.com>, 
	Alex Gaynor <alex.gaynor@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, 
	Gary Guo <gary@garyguo.net>, =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, rust-for-linux@vger.kernel.org, 
	linux-kernel@vger.kernel.org
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On Mon, Feb 5, 2024 at 3:19=E2=80=AFAM Alice Ryhl <aliceryhl@google.com> wr=
ote:
>
> The `byte_sub` method was stabilized in Rust 1.75.0. By using that
> method, we no longer need the unstable `ptr_metadata` feature for
> implementing `Arc::from_raw`.
>
> This brings us one step closer towards not using unstable compiler
> features.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> This patch is based on rust-next because it depends on the patch [1]
> that upgrades to Rust 1.75.0.
>
> [1]: https://lore.kernel.org/all/20231224172128.271447-1-ojeda@kernel.org=
/
>
>  rust/kernel/lib.rs      |  1 -
>  rust/kernel/sync/arc.rs | 10 ++++------
>  2 files changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> [...]
> @@ -239,18 +239,16 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
>          // binary, so its layout is not so large that it can trigger ari=
thmetic overflow.
>          let val_offset =3D unsafe { refcount_layout.extend(val_layout).u=
nwrap_unchecked().1 };
>
> -        let metadata: <T as Pointee>::Metadata =3D core::ptr::metadata(p=
tr);
>          // SAFETY: The metadata of `T` and `ArcInner<T>` is the same bec=
ause `ArcInner` is a struct
>          // with `T` as its last field.
>          //
>          // This is documented at:
>          // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.

The comment should be reworded, no more metadata and no unsafe block
so it doesn't have to be SAFETY.

> -        let metadata: <ArcInner<T> as Pointee>::Metadata =3D
> -            unsafe { core::mem::transmute_copy(&metadata) };
> +        let ptr =3D ptr as *mut ArcInner<T>;

Nit: this could be `.cast::<ArcInner<T>>().cast_mut()` to make the
intentional mutability change clear.

>          // SAFETY: The pointer is in-bounds of an allocation both before=
 and after offsetting the
>          // pointer, since it originates from a previous call to `Arc::in=
to_raw` and is still valid.
> -        let ptr =3D unsafe { (ptr as *mut u8).sub(val_offset) as *mut ()=
 };
> -        let ptr =3D core::ptr::from_raw_parts_mut(ptr, metadata);
> +        let ptr =3D unsafe { ptr.byte_sub(val_offset) };
>
>          // SAFETY: By the safety requirements we know that `ptr` came fr=
om `Arc::into_raw`, so the
>          // reference count held then will be owned by the new `Arc` obje=
ct.
>
> base-commit: f090f0d0eea9666a96702b29bc9a64cbabee85c5
> --
> 2.43.0.594.gd9cf4e227d-goog
>
>

Worth noting that the same change has been in upstream for a while,
since https://github.com/rust-lang/rust/pull/99113.

With the above changed or otherwise clarified:

Reviewed-by: Trevor Gross <tmgross@umich.edu>